1. Field of the Invention
This invention relates to a biaxially oriented laminate polyester film. More specifically, it relates to a biaxially oriented laminate polyester film which has excellent flatness, scraping resistance and take-up property and which is useful as a base film for a magnetic recording medium.
2. Description of the Related Art
A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for a wide variety of applications, especially as a base film for a magnetic recording medium, because of its excellent physical and chemical properties.
In recent years, with attempts made to increase the density and capacity of a magnetic recording medium, an improvement in the surface flatness and a reduction in the thickness of a base film have been strongly desired However, when the surface of a base film is made flat to retain excellent electromagnetic conversion characteristics, slipperiness becomes insufficient. For instance, when it is rolled, wrinkles are produced or blocking occurs, resulting in an uneven surface of a film roll which deteriorates production yield, narrows the suitable ranges of tension, contact pressure and speed at the time of winding up and makes it extremely difficult to wind up the film. When slipperiness is low in a film processing step, friction between a film and a metal roll increases and scraped powders are produced, resulting in omission of a magnetic recording signal, that is, a dropout.
Generally speaking, to improve the slipperiness of a polyester film, there are employed a method for providing an uneven surface to a film (1) by precipitating inert particles from a residual catalyst into a starting material polymer in the production process; (ii) by adding inert particles; or the like. As the size and amount of particles contained in the film increase, the film has more improved slipperiness.
On the other hand, as described above, the surface of a base film is desired to be as flat as possible from the view of improvement of the electromagnetic conversion characteristics thereof. When a film of which the base film surface is rough is formed into a magnetic recording medium, the uneven surface of the base film makes the surface of a magnetic layer formed on the base film uneven, thereby deteriorating the electromagnetic conversion characteristics. In this case, as the size and amount of the particles contained in the base film increase, the surface of the base film becomes rougher and the electromagnetic conversion characteristics thereof becomes worse.
As means for improving both slipperiness and electromagnetic conversion characteristics, which are inconsistent with each other, there is a widely known means for forming a laminate film having a flat surface coated with a magnetic layer to improve the electromagnetic conversion characteristics and an opposite surface having a roughened surface to improve the slipperiness.
However, even when the above biaxially oriented laminate polyester film is used and a surface opposite to its magnetic layer-coated surface is roughened (to be referred to as "rough surface" hereinafter), it still has the problems of slipperiness and scraping resistance and a further problem that, since the thickness of the base film is small, the magnetic layer-coated surface is affected depending on the amount, kind and particle diameter of a lubricant added to the rough surface so that the flat surface has protuberance, thereby causing deterioration in the flatness.
As means for improving abrasion resistance and travelling property, there is proposed a means for containing particles A and an agglomerate of particles B in a base film to improve abrasion resistance and travelling property.
Japanese Laid-open Patent Application No. Hei 3-90329 proposes a method for adding silica particles and linearly agglomerated silica particles. However, when the linearly agglomerated silica particles, i.e., agglomerates of particles are used in combination with the silica particles, the affinity of the particles with a base film is impaired because the particles are agglomerated and they fall off easily by their contact with a device. Furthermore, the particles tend to agglomerate into a big mass to form protrusions which deteriorate the electromagnetic conversion characteristics.
As means for improving abrasion resistance, there is proposed a means for containing single-component particles in a base film. Japanese Laid-open Patent Application No. Hei 4-119845 proposes a method for containing spherical silica particles. However, when particles of small particle size are contained, a gap produced by unevenness on the surface is small and hence, an air squeeze property deteriorates at the time of winding up a film in the production process to cause a defect on the roll. When the particle size is too large, the surface becomes too rough and in consequence, is liable to be scraped, or a flat surface to be coated with a magnetic layer is affected such that the flat surface has protuberance, thereby causing deterioration in the electromagnetic conversion characteristics.